(1) Field of the invention
This invention relates to a method for fabricating a III nitride film, particularly usable as an underfilm for a semiconductor element such as a light-emitting diode or a high velocity IC chip.
(2) Related Art Statement
III nitride films are employed as semiconductor films constituting a light-emitting diode, and recently, win a lot of attention as semiconductor films constituting high velocity IC chips to be used in cellular phone systems.
Such III nitride films are usually fabricated by MOCVD methods. Concretely, a substrate on which III nitride films are formed is set onto a susceptor installed in a given reactor, and then, heated to 1000xc2x0 C. or over with a heater provided in or out of the susceptor. Thereafter, raw material gases are introduced with a carrier gas into the reactor and supplied onto the substrate.
On the substrate, the raw material gases are dissolved through thermochemical reaction into constituent elements, which are reacted to deposit and fabricate a desired III nitride film on the substrate.
As the composition of a III nitride film is changed, the lattice constant of the film is changed on a larger scale. In this case, the difference in lattice constant between the III nitride film and the substrate is enlarged, and thus, more misfit dislocations may be created at the boundary between the III nitride film and the substrate.
Under the condition, if the III nitride film is epitaxially grown, many dislocations of the order of about 1010/cm2 may be created due to the propagation of the misfit dislocations. As a result, the crystal quality may be deteriorated, and thus, the electrical and optical properties of the III nitride film may be deteriorated.
In order to eliminate the above problem, such an attention is made as making a mask of SiO2, etc., on a substrate, and epitaxially growing a III nitride film laterally on the mask. According to the epitaxially growing method, misfit dislocations, which are created at the boundary between the substrate and the III nitride film to be fabricated, are propagated only in the direction parallel to the top surface of the mask, not in the thickness direction of the III nitride film. Therefore, the dislocation density of the III nitride film can be reduced in between the pattern rods of the mask.
In the above fabricating method, however, a lithographing process including a given etching process is required so as to fabricate the SiO2 mask, and thus, the number of steps for fabricating the III nitride film is increased, causing the total fabricating process for the III nitride film to be complicated.
It is an object of the present invention to provide a method capable of easily fabricating a III nitride film of lower dislocation density, and in addition, a substrate for epitaxial growth.
In order to achieve the above object, this invention provides a method for fabricating a III nitride film, including the steps of:
preparing a given base made of a first Al-including nitride,
forming plural island-shaped crystal portions made of a second nitride which are isolated from one another, and
epitatially growing a nitride film made a third nitride from the island-shaped crystal portions functioning as nuclei.
In this invention, the term xe2x80x9cbasexe2x80x9d means a given substrate and a given underfilm which are made of the first Al-including nitride.
The inventors had intensely studied to obtain a III nitride film with lower dislocation density through an easy fabricating process without the need to use, for example, a patterned mask made of SiO2.
FIGS. 1-3 are conceptual views for the fabricating method of the present invention.
First of all, on a base 1 made the first Al-including nitride are island-shaped crystal portions 2-1 through 2-4 made of the second nitride, as shown in FIG. 1. The island-shaped crystal portions 2-1 through 2-4 are employed as nuclei for epitaxially growth. Then, a III nitride film 3A is epitaxially grown from the island-shaped crystal portions 2-1 through 2-4, as shown in FIG. 2, and thus, a III nitride film 3 as desired is fabricated, as shown in FIG. 3.
That is, since the sizes of the island-shaped crystal portions 2-1 through 2-4 are reduced remarkably, the III nitride film 3A is grown three-dimensionally from the island-shaped crystal portions functioning as nuclei in an early growth step. Therefore, dislocations penetrated from the base 1 are curved and propagated laterally. As a result, even though there are many dislocations in the base 1, almost all of the dislocations are not propagated in the direction perpendicular to the surface of the base 1, that is, in the thickness direction of the III nitride film 3.
Accordingly, if the island-shaped crystal portions 2-1 through 2-4 are employed as nuclei for epitaxial growth and then, a III nitride film as desired is epitaxially grown from the crystal portions, the dislocation density of the III nitride film can be reduced. Also, since the dislocation density of the island-shaped crystal portions is very low, the dislocation density of the III nitride film can be greatly reduced.
In the present invention, as mentioned above, since the lateral epitaxial growth is enhanced, the low dislocation density of the III nitride film can be realized through the total growth process beginning with the early growth step.
The above-mentioned tendency becomes conspicuous, on the condition that the full width of half maximum (FWHM) of an X-ray rocking curve at (002) reflection of the base 1, or an underfilm instead of the base 1, is set to 90 seconds or below.
The island-shaped crystal portions can be made by controlling MOCVD conditions appropriately. Three kinds of principles (discussed in the following) are considered as the reasons of such a growth mode.
The first principle relates to the difference in lattice constants between the first Al-including nitride to form the base and the second nitride to form the island-shaped crystal portions. That is, if the lattice constant of the second nitride is larger than that of the first nitride, a given film made of the second nitride is grown on SK mode. Therefore, the film is not formed uniformly, but grown from given nuclei to have an island-like shape. As a result, the above-mentioned island-shaped crystal portions are made.
The second principle relates to the oxidation of the main surface of the base which is made of the first Al-including nitride. That is, if a given oxidized film is formed on the base through the oxidation of the main surface of the base, the second nitride is selectively deposited and agglomerated on exposed areas without the oxidized film of the main surface of the base. As a result, a uniform film is not formed as the second nitride, and is grown three-dimensionally from the exposed areas, which is made of nitride, as nuclei. In this way, the above-mentioned island-shaped crystal portions are made.
The third principle relates to the agglomeration due to composition fluctuation. That is, the constituents of the base is dispersed ununiformly to some degree therein, not uniformly. In this case, the second nitride is selectively deposited and agglomerated, correspondent with the composition distribution in the base. As a result, a uniform film is not formed as the second nitride, and is grown three-dimensionally from the agglomerated portions. In this way, the above-mentioned island-shaped crystal portions are made.
As mentioned above, although the nitride film made of the third nitride is epitaxially grown from the island-shaped crystal portions functioning as nuclei, the nitride film is also epitaxially grown directly on the base to some degree, as shown in FIG. 2.
In the present invention, the term xe2x80x9cthe nitride film made of the third nitridexe2x80x9d means a single nitride layer and a multilayered structure made of plural nitride films such as a distorted superlattice structure. Also, the term means a film of which the composition is continuously or stepwisely inclined in the thickness direction.
This invention relates to a substrate for epitaxial growth, including;
a base made of a first Al-including nitride, and
plural island-shaped crystal portions made of a second nitride which are formed on the base and isolated from one another.
In this invention, the term xe2x80x9cbasexe2x80x9d also means a given substrate and a given underfilm which are made of the first Al-including nitride.
The substrate for epitaxial growth includes the plural island-shaped crystal portions, which are made of the second nitride on the base, which is made of the first nitride. Therefore, if the substrate of the present invention is employed, a III nitride film of low dislocation density can be obtained.
The resulting III nitride film made of the third nitride may be employed as a semiconductor film to construct a semiconductor element, particularly such as a high velocity IC chip and a semiconductor light emitting element such as light-emitting diode. Also, the substrate for epitaxial growth may be employed for such a semiconductor element or a semiconductor light-emitting element. As a result, the luminance efficiency of the semiconductor light-emitting element and the high velocity response of the semiconductor element can be improved.